LNT model

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Linear relationship of the LNT model shown in curve B , A as a supralinear relationship, C with a linear-quadratic relationship and D as a hormesis

The linear no-threshold model or linear non-threshold model (LNT model, German  "linear without threshold value" ) is a model that is used in radiation protection and is used to quantify and quantify exposure to radioactive rays to set regulatory limit values.

properties

The 1959 by the International Commission on Radiological Protection ( English International Commission on Radiological Protection , ICRP) introduced LNT model is based on a purely linear relationship between radiation dose and cancer cases from. In a diagram, the straight line does not start from a threshold value , but runs from a dose of zero to infinity. According to this thesis, there is expressly no threshold below which cancer does not occur. The LNT model not only ignores the possible Strahlenhormesis , but also the ability of cells , DNA damage to repair , as well as by the organism, damaged cells to be removed. These two mechanisms mean that a small dose over a long period is less dangerous than a large dose over a short period of time.

use

The LNT model is most commonly used to calculate the likelihood of radiation-induced cancer . The validity of the model has been proven by epidemiological studies in the high dose range . In contrast, the validity of the model for calculating the health effects of low radiation doses is controversial. In the low dose range, the predictive power of the LNT model is much less reliable. Regardless of this, the model is also used in the low dose range as a basis for decisions with regard to exposure at the workplace or for medical use of radioactive rays. The same applies to far-reaching political decisions, e.g. B. the use of nuclear energy or the establishment of repositories and their potential impact on the health of the population. The basic assumption of the model is that the long-term biological damage caused by ionizing radiation (essentially the carcinoma risk) is directly proportional to the radiation dose. Using a dosimeter , it is possible to add up all radiation exposures without taking the individual dosage or the dose rate (dose rate) into account. According to the LNT model, radiation is always harmful to health without assuming a threshold (the radiation dose) below which no health damage is to be expected. This model also assumes that the sum of many small radiation doses is just as harmful to health as the sum of these doses as a single dose (linear effect).

One of the organizations that plays a key role in the international establishment of recommendations in radiation protection, the United Nations Scientific Committee for Investigating the Effects of Atomic Radiation ( UNSCEAR ), made recommendations in 2014 that, unlike in previous years, no longer validate the LNT Model with low radiation dose. The recommendation states that the multiplication of very low radiation doses by a large number of people in order to estimate the frequency of radiation-induced health effects within a population group should no longer be used. This applies in the event that the sum of low radiation doses is equal to or lower than the natural radiation exposure that everyone is exposed to anyway. In doing so, UNSCEAR clearly moved away from its own earlier recommendations. Regardless of this development due to recent scientific findings, however, z. B. in Germany still adhered to the older recommendations based on the LNT model. So it is called z. B. in a report for the disposal commission of the German Bundestag by Gerald Kirchner from the Center for Science and Peace Research (ZNF) together with Matthias Englert from the Öko-Institut on December 8, 2015: “... is a radioactive material according to the internationally valid principles of radiation protection to be assessed as harmless only if it falls below the legally regulated values ​​for "release" for restricted recovery or unrestricted use. These values ​​are chosen so that the resulting dose falls below the so-called “de minimis dose” by a few tens of microsieverts ”.

The LNT model is currently being contested by various parties that have been submitted to and assessed by the US Nuclear Regulatory Commission (NRC). In a challenge to nuclear medicine doctor Carol Marcus of the US University of UCLA , he describes the LNT model as "scientific nonsense".

The LNT hypothesis, however, was confirmed by the INWORKS study published in 2015 . The criticism formulated by Edward Calabrese in 2011 of the scientific basis of the far-reaching conclusions drawn at the time and thus also the radiation hormesis theory, which is still often used, was not supported by this study. The INWORKS study used a database of over 300,000 workers exposed to radiation to demonstrate a linear relationship between dose and risk, even for small doses. The increase in the risk of death from radiation-induced cancer is 48% per Gray. With an exposure of 10 mGy, the risk of death increases by 0.48%. Note: An absorbed dose of 1 Gray (Gy) corresponds to a radiation exposure (radiation dose) of 1 Sievert (Sv) or 10 mGY = 10 mSv. The INWORKS long-term study on over 300,000 industrial workers in nuclear power plants in the USA, UK and France thus showed that there is a minimally increased risk of developing chronic myeloid leukemia (30 sick people due to additional long-term radiation exposure). The increased risk was found for those study participants who were exposed to a relatively high radiation exposure of 50 to 100 millisievert, but not at lower doses. The result corresponds to the comparably large 15-country study, which also found a minimally increased risk in the low range of radiation exposure for industrial workers in nuclear power plants. Both studies, however, have methodological weaknesses that limit the informative value of the studies and thus do not allow any hasty conclusions at this point in time. B. that the "radiation hormesis theory" has been refuted. Experimental molecular genetic studies support the idea that a low radiation exposure at least leads to a radioadaptive response of the organism, which protects against damage to health. For example, the INWORKS study (and 15-country study) did not take into account the possible radiation exposure of study participants in the context of medical examinations, which is particularly high in the USA and there on average over 3 millisieverts per capita and year. The tobacco consumption of the study participants was also not taken into account, which is of particular importance because this also increases the risk of developing carcinoma (including leukemia) due to the radioactive radon contained in tobacco. These possible "confounders" were discussed by the authors of the studies. It was not discussed, however, that the corresponding disruptive factors (here e.g. exposure to radon from smoking, medical radiation exposure, etc.) severely limit the informative value of epidemiological studies, especially when there is a minimally increased risk (in contrast to a medium to high risk). For example, the Committee on the Analysis of Cancer Risks in Populations near Nuclear Facilities makes the following statement: Without exception, confounding variables are an important aspect in all epidemiological studies - this applies above all to studies that examine the risk of rare diseases with low radiation exposure. Even a small disruptive factor can lead to significant distortions of the results or incorrect results. As a result, this can lead to unjustified or even counterproductive medical instructions or political decisions. In a report from 2013, the European Center for Disease Prevention and Control (ECDC) points to the central communication problem between science, politics and the population in this context: Risk communication of a possible health hazard takes place in an emotionally charged environment. Under such conditions, the usual rules of communication often fail and can make the situation worse (“fall short or can make the situation worse”).

Alternatives

In contrast to the LNT model, there is the threshold model , according to which a very low exposure (<100 millisievert continuous annual total dose) is harmless. For comparison: the mean annual dose in Germany due to natural radiation exposure is 2.1 millisievert, with values ​​of up to 10 millisievert being reached in some regions of the country (especially in the low mountain range) and depending on lifestyle. In some regions of the world there is a much higher radiation exposure without any damage to health in the exposed population has been proven. Due to epidemiological and molecular genetic studies, the LNT model has been questioned. In Chernobyl, the LNT model made a significant contribution to the fact that numerous people panicked and suffered and suffer from psychosomatic complaints for years, which in total caused far more deaths than radiation. In Fukushima only one person (a worker in the nuclear power plant) has become ill with cancer due to radiation and has since died - however, 1,500 people died from evacuation stress as a result of a panic evacuation of the affected area.

history

The LNT hypothesis was developed by Hermann Joseph Muller , who received the Nobel Prize in Physiology or Medicine in 1946 for the discovery that mutations can be caused with the help of X-rays . The hypothesis found experimental support at that time by Curt Stern .

Critical publications on the LNT model

For ethical reasons, current publications on the health effects of ionizing radiation do not use any data obtained from human experiments. As a result, there are hardly any empirically reliable data, especially on the effects of low doses of ionizing radiation, which explains a large part of the controversy surrounding the LNT model. This means that empirical research on the topic is basically only possible on animals or in a retrospective form. Retrospective studies are e.g. B. in areas or apartments with high radioactive pollution of natural or unnatural kind possible. There are also publications on the medical use of X-rays shortly after their discovery, which have been retrospectively evaluated. The following examples with publications from peer-reviewed scientific journals :

Homes in Taiwan contaminated with Cobalt-60

In Taiwan in the 1980s and 1990s, construction steel was accidentally alloyed with Cobalt-60 and used in houses. Cobalt-60 emits hard gamma radiation , which resulted in annual doses of over 500 millisieverts per person in over 1600 buildings, which corresponds to five hundred times the annual limit value permitted in Germany. Over the entire period from the installation to the discovery of the contaminated structural steel, the residents received total doses of up to 4000 mSv, and more than 10,000 people were statistically recorded and subsequently medically examined. As a result, no health or genetic damage occurred in any affected person, nor were there any statistical accumulations of leukemia. Compared to the unaffected population, the affected people were about twenty times less likely to develop cancer, and congenital malformations about ten times less likely.

Radon exposure of the residents of the Iranian city of Ramsar

In Ramsar, comparatively large amounts of the radioactive gas radon diffuse from the soil, which in parts of the city leads to annual radiation exposure of several hundred mSv. Compared to the rest of the population of Iran, there is no statistically relevant increased frequency of genetic damage or cancer cases in Ramsar.

Medical use of X-rays shortly after their discovery

The discovery of X-rays in 1895 led not only to imaging applications, but also to attempts to use the radiation for healing purposes. During this time, numerous medical studies have been published, which have recently been the subject of extensive meta-studies. Again, the LNT model and its predictions were not confirmed.

Individual evidence

  1. ↑ International Commission on Radiation Protection : ICRP Publication 99, Low-dose Extrapolation of Radiation-related Cancer Risk. Annals of the ICRP 35 (4). Elsevier , 2005.
  2. ↑ M. Tubiana, LE Feinendegen, C. Yang, JM Kaminski: The linear no-threshold relationship is inconsistent with radiation biologic and experimental data. In: Radiology. Volume 251, number 1, April 2009, pp. 13-22, doi: 10.1148 / radiol.2511080671 , PMID 19332842 , PMC 2663584 (free full text).
  3. ↑ The 2007 Recommendations of the International Commission on Radiological Protection , International Commission on Radiological Protection , accessed on July 31, 2015
  4. ^ Health Impacts, Chernobyl Accident Appendix 2 , World Nuclear Association, 2009. Retrieved July 31, 2015.
  5. ↑ United States General Accounting Office: Report to the Honorable Pete Domenici, US Senate , June 2000, RADIATIONSTANDARDS, (PDF) .
  6. ^ Commission for the Storage of Highly Radioactive Waste Materials K-MAT 48: Report “Transmutation” p. 117, 118, (PDF) .
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  8. ↑ Marcel Krok: Attack on radiation geneticists triggers furor . In: Science Magazine , October 18, 2011
  9. Jump up ↑ K. Leuraud, DB Richardson, E. Cardis, RD Daniels, M. Gillies, JA O'Hagan, GB Hamra, R. Haylock, D. Laurier, M. Moissonnier, MK Schubauer-Berigan, I. Thierry-Chef, A. Kesminiene: Ionizing radiation and risk of death from leukaemia and lymphoma in radiation-monitored workers (INWORKS): an international cohort study. In: The Lancet. Hematology. Volume 2, number 7, July 2015, pp. E276 – e281, doi : 10.1016 / S2352-3026 (15) 00094-0 , PMID 26436129 , PMC 4587986 (free full text).
  10. ↑ E. Cardis, M. Vrijheid, M. Blettner, E. Gilbert, M. Hakama, C. Hill, G. Howe, J. Kaldor, CR Muirhead, M. Schubauer-Berigan, T. Yoshimura, F. Bermann, G. Cowper, J. Fix, C. Hacker, B. Heinmiller, M. Marshall, I. Thierry-Chef, D. Utterback, YO Ahn, E. Amoros, P. Ashmore, A. Auvinen, JM Bae, JB Solano , A. Biau, E. Combalot, P. Deboodt, A. Diez Sacristan, M. Eklof, H. Engels, G. Engholm, G. Gulis, R. Habib, K. Holan, H. Hyvonen, A. Kerekes, J. Kurtinaitis, H. Malker, M. Martuzzi, A. Mastauskas, A. Monnet, M. Moser, MS Pearce, DB Richardson, F. Rodriguez-Artalejo, A. Rogel, H. Tardy, M. Telle-Lamberton, I. Turai, M. Usel, K. Veress: Risk of cancer after low doses of ionizing radiation: retrospective cohort study in 15 countries. In: BMJ. Volume 331, number 7508, July 2005, p. 77, doi : 10.1136 / bmj.38499.599861.E0 , PMID 15987704 , PMC 558612 (free full text).
  11. ↑ Y. Shibamoto, H. Nakamura: Overview of Biological, Epidemiological, and Clinical Evidence of Radiation hormesis. In: International Journal of Molecular Sciences . Volume 19, number 8, August 2018, p., Doi : 10.3390 / ijms19082387 , PMID 30104556 , PMC 6121451 (free full text).
  12. ↑ Alison Abbott: Researchers pin down risks of low-dose radiation. In: Nature. 523, 2015, p. 17, doi : 10.1038 / 523017a .
  13. ^ Committee on the Analysis of Cancer Risks in Populations near Nuclear Facilities-Phase I; Nuclear and Radiation Studies Board; Division on Earth and Life Studies; National Research Council: Epidemiologic Studies . In: Analysis of Cancer Risks in Populations Near Nuclear Facilities: Phase I. 2012. ISBN 978-0-309-25571-4 .
  14. ↑ ECDC : Technical Report: A literature review on effective risk communication for the prevention and control of communicable diseases in Europe . 2013. (PDF) .
  15. ↑ BfS - How high is the natural radiation exposure in Germany? In: bfs.de. August 10, 2018, accessed May 20, 2019 .
  16. ↑ JJ Cardarelli, BA Ulsh: It Is Time to Move Beyond the Linear No-Threshold Theory for Low-Dose Radiation Protection. In: Dose-response: a publication of International Hormesis Society. Volume 16, number 3, 2018 Jul-Sep, p. 1559325818779651, doi : 10.1177 / 1559325818779651 , PMID 30013457 , PMC 6043938 (free full text).
  17. ^ WHO : Chernobyl: the true scale of the accident. In: who.int. Retrieved May 20, 2019 .
  18. ↑ Fred Pearce: What was the fallout from Fukushima? In: The Guardian . May 1, 2019, accessed May 20, 2019 .
  19. ^ G. Pontecorvo: Hermann Joseph Muller. In: Annual Review of Genetics. 2, 1968, p. 1, doi: 10.1146 / annurev.ge.02.120168.000245 .
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  21. ^ WL Chen et al. "Effects of Cobalt-60 Exposure on Health of Taiwan Residents Suggest New Approach Needed in Radiation Protection, Dose Response", pp. 63-75 (2007), available online, Effects of Cobalt-60 Exposure on Health of Taiwan Residents Suggest New Approach Needed in Radiation Protection
  22. ↑ Popular scientific article [1]
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  24. ↑ JM Cuttler et. al, "Application of Low Doses of Ionizing Radiation in Medical Therapies", Dose Response (2020)